The present invention in general relates to a neutron shield and a cask that uses the neutron shield. More particularly, this invention relates to a neutron shield capable of enhancing the working efficiency by lowering the viscosity in uncured state and maintaining a sufficient pot life, and also maintaining an excellent heat resistance and neutron shielding capacity. Further, this invention relates to a cask that stores the spent fuel assemblies in the neutron shield.
In the background of recent progress in nuclear industries, various nuclear facilities including reactors and fuel reprocessing plants are built around the world, and at these nuclear installations, maximum caution is required to minimize the radiation dose exposed to the human, and avoid loss and damage of structural members and equipment materials due to radiation. The neutrons released from the fuel and spent fuel at nuclear facilities are high in energy, and can pass through material. They generate gamma-rays when they collide with other substance. The radiated gamma-rays may cause serious human casualties and damages of nuclear facilities and materials. As a consequence, neutron shields capable of shielding neutrons safely and securely are being developed continuously.
Concrete is generally used to shield the neutron s. However, when concrete is to be used for such purpose the thickness of the wall has to be made considerably thick. This is a disadvantage in nuclear facilities such as atomic-powered ship because most of them have to light weighted and small and compact. Accordingly, there is a requirement of lightweight neutron shields.
Faster neutrons, among other neutrons, are effectively decelerated when they collide with hydrogen atoms of nearly same mass. Therefore, substance of high hydrogen density, that is, high hydrogen content, can effectively shield the faster neutrons. Accordingly, water, paraffin or polyethylene may be used as the neutron shielding material. Water is lighter in weight than concrete. However, because water is a liquid, it is difficult to handle. Furthermore, the water has to be stored into a container and neutron shielding capability of the material of the container becomes another problem.
On the other hand, it is proposed to form neutron shields by using lightweight materials high in hydrogen content and excellent in neutron decelerating effect, such as paraffin, polyethylene, other polyolefin thermoplastic resins, unsaturated polyester resin and other thermosetting resins, and polymethacrylic acid, either independently or in mixture, or these materials blended with boron compound known to have a wide absorbing sectional area in slow and thermal neutrons, such as paraffin containing boron compound, polyethylene containing boron compound, and ester polymethacrylate containing boron compound.
Recently, a new neutron shield is formed by using epoxy resin, and blending with a huge volume of aluminum hydroxide as refractory, and a trace of boron carbide as neutron shielding material. The epoxy resin is usually a two-part reactive cold-setting epoxy resin consisting of main component and hardener, and the main component is bisphenol A type main component (hydrogen content=7.1% by weight) with epoxy equivalent of 184 to 194 and molecular weight of about 380, and the hardener is aliphatic polyamine, alicyclic polyamine, polyamide amine, and epoxide adduct, which may be used either alone or in mixture.
When forming the neutron shield by using such two-part reactive cold-setting epoxy resin consisting of main component and hardener, in order to obtain a uniform neutron shield by homogeneously mixing the epoxy resin main component, hardener, aluminum hydroxide, and boron carbide, it required long kneading and filling work of about 30 minutes in small units. In this case, since the hardener is contained in the kneaded neutron shield, it may get solidified unless poured in promptly, and the working efficiency is poor because the viscosity is high. That is, owing to high viscosity, the fluidity in the hose is poor when pouring in, and the pouring amount per unit time is small, and still more, because of kneading in small units, the number of times of interruption in the pouring process increases when manufacturing a large-sized neutron shield, and the total pouring process takes much time and labor.
Incidentally, the pot life of the neutron shield mixing such two-part reactive cold-setting epoxy resin varies with the passing of the kneading time, but it is generally 2 hours when the initial temperature is about 30xc2x0 C. in kneading process. This duration of 2 hours includes the kneading and filling time, for example, 30 minutes as mentioned above, and it is demanded to shorten the kneading and filling time by lowering the viscosity. The pot life means, in this case, the duration from the fluid state by kneading until a minimum fluidity necessary for pouring is left over.
On other hand, the aluminum hydroxide contained in the neutron shield mentioned above is high in hydrogen content and is intended to give flame retardant property and neutron shielding capability, but when exposed to high temperature environment for a long time, the hydrogen content declines gradually.
It is an object of this invention to provide a neutron shield capable of enhancing the working efficiency by lowering the viscosity when forming the neutron shield, and maintaining enough hydrogen content for assuring heat resistance and neutron shielding capability even in high temperature environment for a long period after forming the neutron shield. It is another object of this invention to provide a cask that uses this neutron shield.
The neutron shield according to one aspect of this invention has a two-part reactive cold-setting epoxy resin consisting of an epoxy resin adding long-chain aliphatic glycidyl ether epoxy resin as main component, and alicyclic polyamine, polyamide aliphatic polyamine and epoxy adduct as hardener. Since the long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent is used as the main component, the viscosity can be lowered to about 20 to 25 poise, and therefore, the working efficiency is enhanced. Furthermore, the hydrogen content in the main component can be also increased to about 7.5 to 8.5% by weight. By using this main component, a flexible material can be selected for the hardener, as the hardener having favorable effects on the pot life, by using alicyclic polyamine, polyamide polyamine, aliphatic polyamine, or epoxide adduct, either alone or in a mixture of two or more kinds, as the hardener, a sufficient pot life is assured, and the amount of active hydrogen in curing process is increased, and by using alicyclic polyamine, in particular, a two-part reactive cold-setting epoxy resin further enhanced in heat resistance is realized. The pot life can be specifically extended to about 3 to 3.5 hours, for example, when the temperature is about 30xc2x0 C. when kneading the neutron shielding materials containing this two-part reactive cold-setting epoxy resin, and hence the possible pouring time is increased, and massive kneading neutron shielding materials is possible, and the number of times of interruption is decreased in the process of forming a large-sized neutron shield, so that the time and labor required in forming the neutron shield may be substantially saved.
The neutron shield according to another aspect of this invention has a two-part reactive cold-setting epoxy resin consisting of an epoxy resin adding long-chain aliphatic glycidyl ether epoxy resin as main component, and alicyclic polyamine, polyamide polyamine, aliphatic polyamine and epoxy adduct as hardener, a refractory composed of aluminum hydroxide or magnesium hydroxide, and a neutron absorbing material. Pyrolysis temperature of aluminum hydroxide for inducing massive moisture release at high temperature is generally 245 to 320xc2x0 C., whereas the dehydration pyrolysis temperature of magnesium hydroxide is 340 to 390xc2x0 C. Since magnesium hydroxide is used in part or whole of the refractory for composing the neutron shield, the heat resistance of the neutron shield in high temperature environment is enhanced.
The cask according to still another aspect of this invention uses the neutron shield described above. The cask further comprises plural square pipes having neutron absorbing capability inserted in a cavity of a shell main body for shielding gamma-rays, shaping according to the outer shape of a basket of square sectional shape formed by the square pipes, and containing and storing spent fuel assemblies in each cell of the basket inserted into the cavity. Since the long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent is used as the main component, the viscosity can be lowered to about 20 to 25 poise, and therefore, the working efficiency is enhanced. Furthermore, the hydrogen content in the main component can be also increased to about 7.5 to 8.5% by weight. By using this main component, a flexible material can be selected for the hardener, as the hardener having favorable effects on the pot life, by using alicyclic polyamine, polyamide polyamine, aliphatic polyamine, or epoxide adduct, either alone or in a mixture of two or more kinds, as the hardener, a sufficient pot life is assured, and the amount of active hydrogen in curing process is increased, and by using alicyclic polyamine, in particular, a two-part reactive cold-setting epoxy resin further enhanced in heat resistance is realized. The pot life can be specifically extended to about 3 to 3.5 hours, for example, when the temperature is about 30xc2x0 C. when kneading the neutron shielding materials containing this two-part reactive cold-setting epoxy resin, and hence the possible pouring time is increased, and massive kneading neutron shielding materials is possible, and the number of times of interruption is decreased in the process of forming a large-sized neutron shield, so that the time and labor required in forming the neutron shield may be substantially saved.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.